This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Two dimensional (2D) layered materials like graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs) are receiving significant attention across all scientific disciplines due to their unique electrical, mechanical, thermal and optical properties. High carrier velocity, exceptional mechanical stability and near invisibility of graphene has already resulted in its commercialization as stretchable and transparent electrodes and interconnects. Graphene has also been substantially investigated as an alternative to silicon for beyond complementary metal-oxide semiconductor (CMOS) nanoelectronics. However, the absence of a sizeable bandgap prevents the use of graphene in logic circuits and has paved the way for the exploration of semiconducting transition metal dichalcogenides (TMDs) including, but not limited to, MoS2, WSe2, and MoSe2. Several high performance field effect transistors (FETs) based on TMDs have been demonstrated. Various studies also indicate the potential of TMDs for optical, mechanical, chemical and thermal applications. Finally, h-BN complements both, highly conductive graphene and semiconducting TMDs, not only by being a large bandgap insulator, but also, often as a substrate with better interface qualities. Integrating the unique properties of these different 2D materials, therefore, provides numerous possibilities to shape the future of nanoelectronics.
One of the most promising applications of optimally stacked 2D materials is as thin film transistors (TFTs). The recent outburst of the display technology has made it even more appealing since the light emitting diodes (LEDs) and liquid crystal displays (LCDs) are driven by TFTs. TFTs are also used in RFID tags, flexible electronic devices and for sensing applications. Although, the thin film transistor industry is reasonably mature, it is nowhere close to the ultimate potential due to limited material choices. Amorphous silicon (a-Si) is the most popular and widely used material for the TFTs, but the mobility of a-Si is in the range of 0.5-1 cm2/Vs. However, the mobility is still found to be less than 1 cm2/Vs for most cases. Metal oxide semiconductors such as indium tin oxide (ITO), ZnO and most recently alloys such as GaInZnO (GIZO) have demonstrated mobility values as high as 1-100 cm2/Vs, but, the oxide TFTs suffer significantly from threshold voltage shift and hence, electrical instability, due to doping created by oxygen vacancies. Nanowire and carbon nanotube based TFTs have also demonstrated mobility values in the range of 10-100 cm2/Vs. However, the placement of the wires/tubes and the variability in their transport properties depending on their dimensions (diameters) and connectivity (percolation path in a film) are the major challenges in the realization of TFTs using these materials. Therefore, the search for better materials for TFTs continues.
The most desirable features of TFTs are high carrier mobility, high ON-OFF current ratio, low contact resistance, presence of both electron and hole conduction, high optical transparency, temperature stability and mechanical flexibility. 2D layered materials are a natural choice for the TFTs in order to meet these requirements. Moreover, their inherent electrostatic integrity allows them to operate at low power and also make them more scalable.
A need exists for improved technology, including technology that may address the above described disadvantages. In particular, a need exists for improved technology that addresses problems including, but not limited to: 1) low carrier mobility of amorphous silicon TFTs, 2) poor on-off current ratio and low mobility values of organic TFTs, 3) threshold voltage instability of oxide TFTs, 4) variability and placement issues of nanowire and nanotube TFTs, and 5) ability to build TFTs on flexible and optically transparent substrates.